The present invention relates to a catalyst system comprising at least two catalyst zones tailored to remove calcium and sodium from a hydrocarbon feedstock, and a process using this system. More particularly, the first zone of the catalyst system effectively removes calcium and oil-insoluble sodium while the second catalyst zone effectvely removes the oil-soluble organic sodium present in the hydrocarbon feedstock located to protect other catalysts. The process which uses this catalyst system comprises passing a calcium and sodium containing hydrocarbon feedstock over the catalyst system at hydrodemetalation conditions.
Most heavy crudes contain significant amounts of organic metal compounds such as nickel and vanadium. Some are present as insoluble salts which can be removed by conventional filtrating and desalting processes. Yet most of them are present as oil-soluble organometallics which are not removed and continue on to the catalyst bed. They create problems for refiners by depositing just below the external surface of the catalyst particles. As a result, they block the catalyst pore openings and deactivate the catalyst.
A variety of schemes to remove the oil-soluble nickel and vanadium arganometallics from petroleum feedstocks have been suggested. One approach is to frequently replace the fouled catalyst, but this is wasteful and results in costly under-utilization of the catalyst. In recent years, workers in the field have developed hydrodemetalation (HDM) catalysts to protect the more active hydrodesulfurization, hydrodenitrification, or hydrocracking catalysts. Generally, the HDM catalyst contacts the contaminated feed and the metals are deposited before the feed continues through the catalyst bed contacting the active catalysts. In particular, complicated schemes of grading varieties of catalysts which differ in pore size, support composition, and metals loading can result in more efficient use of the individual catalysts.
Most grading schemes involve contacting the hydrocarbon feedstock with a catalyst having large pores designed for metals capacity followed by catalysts with smaller pores and more catalytic metals to remove sulfur and other organic metals. In this way the contaminated feed initially contacts a less active catalyst, thereby allowing the feed to penetrate the catalyst more fully before metal deposition occurs. As the less contaminated feed continues through the catalyst bed, it contacts more active catalysts which promote the deposition of sulfur and other organic metals. Thus, for any given feedstock containing metals that penetrate to the interior of the catalyst, such as nickel and vanadium, there will be an ideal grading of catalyst which will result in the the most efficient use of these catalysts from the top of the reactor to the bottom.
A more complex problem is encountered when iron is present in the petroleum feedstock. It is present either as an oil-soluble organometallic or as an inorganic compound such as iron sulfide or iron oxide. In contrast to nickel and vanadium which deposit near the external surface of the catalyst particles, it deposits preferentially in the interstices, i.e., void volume, among the catalyst particles, particularly at the top of the hydrogenation catalyst bed. This results in drastic increases in pressure drop through the bed and effectively plugs the reactor.
In general, there are two approaches to solving the problem of oil-soluble and oil-insoluble iron deposition on the outside layer of the catalyst particles. One approach, that is somewhat effective for both types, is to control the amount of catalyst of a given size per unit volume of interstitial void volume. The object is to grade the catalyst bed with progressively smaller catalysts so as to provide a decreasing amount of interstitial void volume down the bed in the direction of oil flow. Thus the bed is tailored so as to provide more interstitial volume for iron deposits at the top of the bed than at the lower part of the bed. Hydrogenation catalysts of the same composition may be used throughout the bed; but their particle size or shape is varied from top to bottom of the bed to provide decreasing interstitial voidage volume along the normal direction of oil flow throuh the bed.
Another approach, directed to the problem of oil-soluble, organic iron deposition is to vary the amount of active hydrogenation catalyst present through the catalyst bed. The object is to increase hydogenation catalytic activity through the bed along the direction of feed flow by varying the composition of the crystalline structure of the catalyst. For example, the initial zones of catalyst contained less catalytic metals than subsequent zones. By gradually increasing catalyst activity, zone by zone, iron deposition is distributed throughout the bed. This minimizes the localized loss of voidage and therefore reduces pressure drop buildup.
Previous workers in the field have disclosed other graded catalyst systems for demetalation and desulfurization. For example, U.S. Pat. No. 3,663,434 to Bridge demetalates then desulfurizes using a graded catalyst bed ahead of a desulfurization catalyst bed. U.S. Pat. No. 3,696,027 to Bridge also demetalates and desulfurizes using a catalytic system comprising graded catalyst beds. The beds are graded to contain relatively high-macroporosity catalyst particles followed by low macroporosity catalyst particles, and relatively low hydrogenation activity catalyst particles followed by high hydrogenation catalyst particles.
Accordingly, the term "graded" is used in the art and is used herein to connote that a particular HDM catalyst bed is composed of different types of catalyst particles with differing metals capacities and hydrogenation activities to provide a gradual change through the catalyst system in the direction of feed flow. Thus, a given bed may consist of several different types of catalyst particles in terms of physical properties and chemical composition. Also, we use the term "metals capacity" to mean the amount of metals which can be retained by the catalyst under standardized conditions.
The term "macropore" is used in the art and is used herein to mean catalyst pores or channels or openings in the catalyst particles greater than about 1000 .ANG. in diameter. Such pores are generally irregular in shape and pore diameters are used to give only an approximation of the size of the pore openings. The term "mesopore" is used in the art and used herein to mean pores having an opening of less than 1000 .ANG. in diameter. Mesopores are, however, usually within the range of 40-400 .ANG. in diameter.
Conventional processes, which remove nickel, vanadium, and iron, generally have decreasing macroporosity and increasing mesoporosity in the direction of feed flow through the graded bed. Previous workers found macroporosity to be strongly related to the capacity of catalyst particles to retain metals removed from a hydrocarbon feed contaminated with nickel, vanadium, and iron. In the later catalyst zones, predominantly mesoporous catalysts are preferred. These catalysts have been found to have substantially higher catalytic activity for hydrogenation compared to catalysts having lower surface areas and substantially a macroporous structure. Thus, these two phenomena can be exploited to successfully remove nickel, vanadium, and iron from heavy feedstocks in a graded catalyst system.
The complexity of the problem is again increased when metals such as calcium or sodium are present in the hydrocarbon feedstock. These metals exist in a variety of forms. They typically exist as metal oxides, sulfides, sulfates, or chlorides appearing as salts of such metals. But they can also be present as oil-soluble organometallic compounds, including metal naphthenates. The present invention particularly addresses this, the most complex, metal contaminant problem.
Conventional desalting techniques easily identify and remove the oil-insoluble metallic calcium and sodium salts. If not removed, they deposit interstitially and cause rapid pressure drop buildup. But we know the soluble organometallic compounds with less certainty. We cannot remove these calcium and sodium compounds by conventional methods. Moreover, catalyst systems, like those described above, which are effective for the removal of iron, nickel, and vanadium are unable to control the deleterious effects of oil-soluble calcium and sodium deposition.
In general, we have found that calcium deposits preferentially in the void volume among the catalyst particles. This greatly increases pressure drop through the bed and results in enormous reactor inefficiencies. In addition, we have found that sodium surprisingly behaves in a manner unlike any other metal encountered thus far. In particular, it deeply penetrates the catalyst particles. So the calcium deposits increase the pressure drop through the catalyst bed while the sodium works to block the active sites within the catalyst particles and deactivates them. As a result of our work, it has become clear that we cannot use conventional graded systems successfully to remove calcium and sodium from a hydrocarbon feedstock containing both of these metals. Thus, it is necessary for us to devise a graded catalyst system, taking into consideration such factors as shape, size, porosity, and surface activity of the catalyst particles that successfully removes both calcium and sodium from the hydrocarbon feedstock. Accordingly, it is an object of this invention to provide such a system.